1. Field of the Invention
The present invention relates to a printing apparatus, and more particularly to a printing apparatus in which a printing sheet is advanced by the rotation of a platen and the printing action is conducted while said platen is stopped.
2. Description of the Prior Art
In the conventional printers such as typewriters there is widely employed a structure in which the printing sheet is advanced by the rotation of a platen with a paper advancing motor, and said platen is locked still during the printing action.
FIGS. 1 and 2 illustrate an example of such conventional printing apparatus.
A printing sheet 2 is wound around a platen 1 and maintained thereon without slack with pressure rollers 3, 4. A paper advancing motor 5, composed of a stepping motor for ease of control and for lower cost, transmits rotative motion to the platen 1 through a pulley 6, a belt 7 and another pulley 8.
A detent gear 9, provided with triangular teeth along the outer periphery, is supported on a shaft for the platen 1 and the pulley 8. A detent lever 10 is rotatably supported by a shaft 11 and is biased by a spring 12 in such a manner that a front end 10a of said lever is pressed against a stable point 9a between triangular teeth.
FIG. 2 is a lateral view seen from a direction A shown in FIG. 1, wherein a peripheral length l of the platen corresponds to a length advanced by a step or several steps of the paper advancing motor 5. Upon energization of said motor 5, the rotating force is transmitted to the detent gear 9 through the pulley 6, belt 7 and pulley 8, but said detent gear rotates in a direction B only when the resulting torque exceeds the pressing force of the spring 12 with which the front end 10a is pressed between the triangular teeth of the detent gear 9. Upon said rotation the front end 10a is pushed away in a direction C and then engages with a neighboring stable point 9b. Then the energizing current to the paper advancing motor 5 is terminated, and the platen 1 thus stops the rotation and becomes locked.
As explained in the foregoing, the constant contact of the front end 10a of the detent lever 10 with the triangular teeth of the detent gear 9 in the conventional mechanism gives rise to noise and abrasion and requires a large torque in the paper advancing motor 5. Also in case the printing is to be made at a position different from the regular printing positions defined by the length l, the obtained prints become positionally unstable due to the presence of the above-described detent mechanism.